I. Field of the Invention
The present invention pertains generally to the field of aerosol metrology, and more particularly to an aerosol dilution device capable of providing traceable measurements of the dilution factor and hence aerosol concentrations that are lower or higher than the calibrated range of the particle counting device.
II. Discussion of the Background
The measurement of aerosol mass or number concentration is performed with particle counting devices that measure aerosol concentration over a finite dynamic range. When particle counters measure aerosol concentrations above their operating range, the detectors can become saturated or exhibit undesirable nonlinear behavior. The drawback from these measurement devices arises from the fact that the need to accurately measure aerosol concentrations above the instrument's dynamic range is frequently encountered in practice.
To overcome this obstacle, dilution techniques were employed where the aerosol concentration was diluted by separating the flow path of an aerosol into a filtered path and a non-filtered path. In the filtered path, a filter absorbed the aerosol particles to create relatively pure air which was then mixed with the unfiltered aerosol to create a diluted mix.
The diluted mix was then channeled to a particle counter whereafter the data was extrapolated to arrive at a particle concentration. Furthermore, the magnitude of the dilution factor created by the diluter is not measured traceable to national standards thereby making it impossible to know the accuracy of the dilution.
An example of this prior art technique is demonstrated in FIG. 1. Undiluted aerosol arrives at inlet 10 where it is separated into a filtered path 12 and an unfiltered path represented by capillary tube 14. Capillary tube 14 extends through a felt washer 16. The filtered path 12 leads to HEPA filters 18 which remove the particles in the aerosol to create pure air. The pure air exits the filtered path in the proximity of mixing cone 20 where the pure air is mixed with the undiluted aerosol which has traveled through capillary tube 14. A pressure gauge 22 is situated so as to measure an aerosol path differential pressure and a pressure gauge 24 is situated so as to measure a total path differential pressure. A pressure adjustment mechanism 26 is provided in the filtered path 12 to adjust the pressure of the pure air as it leaves the filtered path. The mixture of pure air and aerosol travels through outlet 28 onward to a particle counter (not shown).
Even when the above technique is employed, the results cannot be considered to provide true traceable particle concentrations (at either low or high levels of concentration) because there is no measurement of the dilution factor by the diluter that is made traceable to national standards through such quantities as flow rate and/or aerosol concentrations.
The traceable calibration of aerosol number concentration has been done in the past by having an aerosol electrometer primary standard measure the electronic current on the airflow and using this data to arrive at a given aerosol concentration that is traceable to national standards through measurement of electronic current, flow rate and the elementary charge on an electron. The drawback from this technique stemmed from the fact that the electrometer was unable to provide accurate data when the aerosol concentration dropped below thousands of particles per cubic centimeter. In practice the need to perform accurate measurements traceable to national standards of aerosol concentrations less than ten particles per cubic centimeter arises, and hence a dilution system that provides quantitative measurements of the dilution factor is needed to measure lower aerosol concentrations that are traceable to national standards.